Network Rail engineering works mean that buses will replace some Southern and Thameslink trains.
Trains between Horsham and Three Bridges will be affected on Sunday.
The engineering works will take place from Saturday August 29 to Monday August 31 between South Croydon and East Grinstead, closing the East Grinstead and Uckfield lines.
Replacement buses will run between Purley and East Grinstead, calling at Upper Warlingham and Oxted; Upper Warlingham and Woldingham; Oxted and East Grinstead, calling at all stations; Oxted and Crowborough, calling at all stations; Crowborough and Uckfield, calling at all stations.
Railway bosses say that replacement buses will not call at Sanderstead or Riddlesdown, where customers should use Transport for London buses or an alternative station. Passengers should change at Purley or East Croydon for trains to and from London.
Tickets will also be accepted on Southeastern services between Tunbridge Wells and London Bridge.
Thameslink services that usually run between Rainham in Kent and Luton or West Hampstead/Kentish Town will run only between Rainham and Dartford.
Passengers should use Southeastern’s amended services between Dartford and London, which include replacement buses between Dartford and Sidcup/Slade Green.
On Sunday August 30, buses will replace trains between Three Bridges and Horsham, where drainage renewal work is taking place.
Also on the Sunday, services that normally run between Sutton and St Albans/Luton will run only between Sutton and Blackfriars.
Chris Fowler, customer services director for Southern and Gatwick Express, said: “We thank customers whose journeys are affected by these vital infrastructure improvements for their patience over the bank holiday weekend.
“We’ll be continuing to help and encourage everyone to travel safely. As well as wearing a face covering, maintaining social distancing and keeping hands clean, please check your route in advance to avoid the busiest times, buy tickets in advance if possible, and allow more time for your journey.”
John Halsall, route managing director at Network Rail southern region, said: “This August bank holiday weekend, thousands of rail workers will be working round the clock to deliver crucial upgrades to the rail network. This will lead to faster, better services and help relieve overcrowding to respond to the huge growth on Britain’s railways.”
More details of how journeys will be affected are available at www.southernrailway.com and www.thameslinkrailway.com.